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1. the OPA2134 Datasheet says to use low ESR ceramic caps with a value of 10nF on the power pins. Any particular reason this is using .1uF film instead other than they were handy?

2. regarding the combo headphone amp, it looks pretty similar to a CMOY design but without the high pass filter and the output resistor placed after the feedback loop. CMOY tweaks I've read about on tangentsoft say to avoid the output (R5 in that schematic) resistor if you can unless you hear hum but like I said, in that design it occurs before the feedback loop so is attenuating the gain a bit? And what about the .22uF cap before the volume knob? Is that a DC offset blocker? Why .22 instead of the original .47?

Hi, The value of the capacitor is largely a judgement call. A 10nF low esr cap will usually do, but I have seen values as large a 1uF. I have lots of 0.1uF polys around for just such use and they work fine. There is no cost difference that matters.

On the head phone part...almost all op amp based such circuits are quite similar. This design is not based on the Cmoy. The 47 ohm resistor is there to protect the IC from excessive loading and not for hum issues. If you use the suggested power supply configuration there will be no power supply hum. Any that exists after that is either from the source or a wiring/layout/construction issue. If you know that your load will not drop significantly below about 60 ohms (higher impedance is really desirable) then you can omit the resistor. It helps insure that the output is not loaded too much thus causing issues with the feedback part of the circuit. In the plans the blocking capacitor is a 2.2uF not .22. It is needed as I chose not to prevent the ICs from responding at DC but wanted exceptional low frequency response. With a good symmetrical supply and good op amps the DC offset at the outputs is quite low but not zero. Feeding it into the second stage would cause it to be amplified and create a larger DC offset there. Since I didn't want a capacitor there it was necessary to insure the off set was small. I figured it was a good trade off. More linear response in the deep bass and fewer components. Other designs use blocking caps in the input connections to ground on each IC. They are usually moderately large non-polar electrolytics. I am not fond of such caps (electrolytics) for audio use and try to avoid their use when possible. It is just how I design things...all designers have ideas of what they want to accomplish and how they want to do it. It makes diy interesting.

Hi Mark, I have tried both types, but the carbom film ones seem a little bit smoother. For those that need a little more zing, use metal film ones. In either case use quality poly coupling caps and ones in the feed back loop.

G'day all, this is interesting as I would never use anything but metal film resistors in high grade audio projects for noise reasons. I think it a bit unfortunate as some builders will use carbon resistors for their sonic 'warmth'. Each to their own I guess. Regards, Felix.

I did my earlier builds with carbon film, the later with metal. I went back to carbon in the signal lines as too many people complained the sound went "dark and cold" before they knew what type of resistors I used.

I now do a mix, keeping metal film to a minimum in tube builds. Yageo pink metal film types sound better than the usual metal film you see in everything nowadays.

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